Electrochemical devices, including proton exchange membrane fuel cells, sensors, electrolyzers, chlor-alkali separation membranes, and the like, are typically constructed from a basic unit called a membrane electrode assembly (MEA). In an MEA of a typical electrochemical cell, an ion conducting polymer electrolyte membrane (PEM) is in contact with a cathode and an anode, and transports ions that are formed at the anode to the cathode, allowing current to flow in an external circuit connecting the electrodes. The PEM is a solid electrolyte which must conduct the appropriate ions and should not conduct electrons or allow passage of reactant gasses. In addition, the PEM should have the necessary structural strength under operating conditions.
Commonly used PEM materials include Nafion.TM. polymers (DuPont Chemicals, Inc., Wilmington, Del., available from ElectroChem, Inc., Woburn, Mass., and Aldrich Chemical Co., Inc., Milwaukee, Wis.) which are perfluorocarbon polymers having attached --SO.sub.3.sup.- anion groups. However, membranes of Nafion alone are inherently weak, especially as the level of hydration increases. Higher strength may be achieved by increasing the thickness of the membrane or by raising the equivalent weight, but at the cost of higher ionic resistance.
One means of constructing a reinforced membrane is to imbibe or infuse an ion-conductive material into a porous inert reinforcing membrane to make a composite membrane. U.S. Pat. No. 5,547,551 (W. L. Gore & Associates, Inc.) describes a composite PEM composed of a porous membrane of polytetrafluoroethylene (PTFE) whose pores are fully impregnated with Nafion.TM. ionomer. Such composite membranes may exhibit separation between the ionomer and the material of the membrane, especially after repeated hydration or heating, and may then develop gaps.
European Patent No. 0,094,679 (Asahi Glass Co. Ltd.) discloses a PEM made by mixing fibrils such as PTFE fibrils into a fluorinated ion exchange resin, extruding the resin as a membrane, cooling the membrane and then stretching it at a specified reduced temperature.